SEQUENCE STRATIGRAPHY OF THE MIDDLE JURASSIC GYPSUM SPRING AND PIPER FORMATIONS IN THE EASTERN BIGHORN BASIN OF WYOMING

The Jurassic of Western North America is divided by a series of regional unconformities, J-0 through J-5. In northern Wyoming and southern Montana, strata between the J-1 and J-2 unconformities, the Gypsum Spring and Piper formations, represent the earliest deposition of the Nevadan Orogeny. In this study, the sequence stratigraphy of these units was examined along the eastern flank of the Bighorn Basin. In addition to the J-1 and J-2 unconformities, two additional sequence boundaries are recognized here. The J-1a separates the lower and upper portions of the Gypsum Spring Formation, and the J-1b separates the Gypsum Spring from the overlying Piper Formation. The first sequence is characterized by alternating thick gypsum beds and red siliciclastic mud, and represents a mosaic of salina and mudflat lithofacies during the TST. Overlying gypsiferous red to chocolate siliciclastic mud represents peritidal to supratidal mudflat deposition during the HST. The J-1a sequence boundary is marked by a purple mudstone which caps the first sequence, and an abrupt deepening into carbonates of the second sequence. This second sequence contains a thick carbonate TST overlain by a thicker carbonate HST. Carbonate facies in this sequence are dominated by laminated lime mudstone, peritidal fine-grained dolostone, stromatolitic and thrombolitic buildups, and peloidal-skeletal packstone to wackestone. The presence of shoals and smaller sub-basins complicates the facies model from a simple onshore-offshore succession. The J-1b sequence boundary is marked by a chert horizon in depositionally updip areas to the south, and abrupt deepening to an ooid grainstone and peloidal packstone in depositionally downdip areas to the north. The J-1b surface is overlain by maroon mudstone of the Piper Formation, and records deposition on a muddy tidal flat. The lack of recognizable flooding surfaces in this third sequence suggests aggradational stacking. This refined sequence stratigraphy will aid in regional correlation and in of the early development of this retroarc foreland basin.